Holographic Measurement and Improvement of the Green Bank Telescope Surface

TL;DR

This paper details the development and application of a 12 GHz holography system on the Green Bank Telescope, which improved the primary mirror surface accuracy, enhanced aperture efficiency, and provided valuable insights into panel distortions affecting high-frequency observations.

Contribution

It introduces a novel holography system integrated with the GBT that significantly reduces surface errors and improves telescope performance at high frequencies.

Findings

Surface error reduced from 390 to 240 microns rms

Aperture efficiency improved at 43 GHz and 90 GHz

Beam pattern measurements show systematic panel distortions

Abstract

We describe the successful design, implementation, and operation of a 12 GHz holography system installed on the Robert C. Byrd Green Bank Telescope (GBT). We have used a geostationary satellite beacon to construct high-resolution holographic images of the telescope mirror surface irregularities. These images have allowed us to infer and apply improved position offsets for the 2209 actuators which control the active surface of the primary mirror, thereby achieving a dramatic reduction in the total surface error (from 390 microns to ~240 microns, rms). We have also performed manual adjustments of the corner offsets for a few panels. The expected improvement in the radiometric aperture efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The improvement in the telescope beam pattern has also been measured at 11.7 GHz with greater than 60 dB of dynamic range. Symmetric features in the beam pattern have emerged which are consistent with a repetitive pattern in the aperture due to systematic panel distortions. By computing average images for each tier of panels from the holography images, we confirm that the magnitude and direction of the panel distortions, in response to the combination of gravity and thermal gradients, are in general agreement with finite-element model predictions. The holography system is now fully integrated into the GBT control system, and by enabling the telescope staff to monitor the health of the individual actuators, it continues to be an essential tool to support high-frequency observations.